The present invention relates to double-walled panels of transparent synthetic material for use in greenhouses and for similar purposes, particularly a double-walled panel made of transparent at least two parallel, separated sheets which are interconnected by means of ribs, with opposite open ends of the sheets being closed by lateral flanges formed by joining the sheets. It is common to refer to acrylic panels if the panels are made of polyacryl material, or to carbonate panels if the panels are of a polycarbonate material. In this text, these sheets will consistently be referred to as double-walled panels to draw attention to the internal construction.
In recent years it has become more and more common artificially to supply carbon dioxide to greenhouses to stimulate plant growth.
However, to have the full benefit of this procedure, the greenhouse cannot be ventilated. On hot sunny days, however, this will unfortunately result in a leaf temperature which exceeds the optimal temperature for plants, whereby the growth of the plants will be reduced. Consequently, the carbon dioxide cannot be utilized effectively in this way.
It is therefore known to fit vents in the roof of the greenhouses and to employ some kind of blind to prevent heat build-up in the greenhouse. However, if the vents in the roof are opened, the supply of carbon dioxide must be cut off. If an attempt is made to reduce the temperature in the greenhouse by shading the light surface, so much light will be removed that this will inhibit the growth of the plants. In other words, this is a vicious circle as either costs increase or growth is inhibited.
Several methods of solving the above problem have been suggested. One of these methods, which is relevant to the present invention, is to use a liquid medium, e.g. water, to filter off the infrared rays in sunlight.
Norwegian Pat. No. 118,047 describes a construction for use with a flat greenhouse roof, where a layer of water covers the transparent roofing material. However, greenhouses are not usually built with flat roofs in countries with snow fall in the winter, and therefore this construction is not suitable in these countries.
If double-walled panels of transparent synthetic material are used for greenhouses with sloping roofs, these make suitable sun filters when the channels between the ribs are filled with a liquid medium. Such use of double-walled panels is mentioned in West German Pat. Nos. 2,500,015; 2,637,914; 2,719,273 and 2,850,264, in which the open ends of the panel are closed by manifold tubes or manifold boxes supplying the liquid.
However, in practice it is extremely difficult to seal properly the connections between the manifold pipe and the open ended panel. The purpose of the present invention is therefore to provide a construction which eliminates the considerable weaknesses experienced by the use of manifold tubes or manifold boxes according to the above-mentioned West German patents.